Do UV islands scaled to 0 have anything applied to those surfaces?
If those islands are used to map a texture, they will map a solid color (representing one pixel of the texture).
Such technique can be used to create pixelation.

If they aren't used to map any texture, it doesn't matter.
Then I'd reccomend scaling to 0,0001 instead of 0, so they can be scaled back up if needed :).